Surface mount technology (SMT) is used for constructing electronic circuits where the components (surface-mounted components/SMCs) are mounted directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface mount device (SMD). SMT has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.
An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.
Edge emitting lasers have beams that widen very fast in a direction normal to the mounting surface. Edge emitting lasers can be mounted in packages with leads that orient the laser perpendicular to the main driver board. The package must have a window cap or encapsulation to protect the laser which is placed at the edge of the lead frame or pedestal of the package base. When mounted on a large, flat surface, such as directly on the main driver board or on a secondary substrate which could be a PCB, ceramic or other substrate that is then mounted on the main driver board, at least a portion of the beam may intersect the surface. To overcome this limitation, edge emitting lasers are often mounted at the edge of a driver board or secondary substrate, where the edge of the laser is adjacent to or nearly adjacent to the edge of the board or substrate so the beam widens in an area beyond the board or substrate. Such an arrangement has several disadvantages. For example, the facet of a laser is typically sensitive to contamination and humidity, among other sensitivities, and the proximity to the edge of the board or substrate may increase the vulnerability of the facet. In addition, the necessity of locating the laser at the edge of a board or substrate limits the flexibility of board design, and may make certain packaging options impractical, for example, certain chip carriers, such as a laminate leadless carrier (LLC). A laminate leadless carrier uses flat metal pads that make contact with a printed circuit board. There are no pins extending out of the package and it may be mounted on the printed circuit board directly. A laminate leadless carrier includes multiple layers of conductive and dielectric layers laminated together.
While SMT provides advantages in manufacturing and circuit layout, the limitations of semiconductor edge emitting laser chips have provided a challenge to SMT lasers. Besides the logistical problems related to positioning a semiconductor laser chip at the edge of a surface mount package, thermal dissipation considerations may conflict with surface mount features. In particular, SMT generally makes it difficult to conduct heat generated by a semiconductor laser chip to the surface of the package, where it may be further dissipated, for example using heat sinks or convection methods.
Photonic semiconductor devices are electrical-to-optical or optical-to-electrical transducers that convert electron signals to photon signals, and photon signals to electron signals. Some photonic semiconductor devices are light emitters such as lasers and light-emitting diodes (LEDs), while others are light detectors such as PN photodiodes, phototransistors, PIN photodiodes, avalanche photodiodes (APDs), single-photon avalanche diodes (SPADs), Silicon photomultiplier (SiPMs), and charge-coupled devices (CCDs). Typical applications for photonic semiconductor devices include telecommunications, range-finders, medical imaging, scientific instruments, and astrophysics applications.
The integration and packaging of photonic semiconductor devices shares many common challenges with its counterparts in integrated circuits (IC) and micro-electromechanical systems (MEMS) such as electrical, thermal and stress issues. There are specific characteristics and challenges related to photonic semiconductor devices.
From structural design perspective, most photonic semiconductor devices have large active area with feature dimension up to a few centimeters, and a functional layer depth that can be as thick as the chip or wafer which is up to a few hundred microns. The contacts are at both front and back sides in most cases. For PIN, APD, SPAD and SiPM devices, the supply voltage may be as high as a few hundred volts. In general, photonic semiconductor devices need optical coupling and/or blocking capabilities, such as antireflection coating (ARC), and filtering. In addition, the assembling of photonic semiconductor devices generally requires precise mechanical dimensions and alignment.
From fabrication process perspective, the assembling lines for photonic devices run at relatively low throughput, and ten thousand parts per year is considered as volume production. Typical production orders range from a few devices to a few thousands of them. The wafer processing lines run two to six inch wafers that are much smaller in size compared to six to twelve inch wafers in IC industry. Heterogeneous semiconductors such as Silicon and III-V are often integrated in the same product.
Photonic semiconductor devices are considered as a specialty item in semiconductor industry when compared with ICs. However photonic semiconductors also face cost reduction pressure from both commercial and military market segments. Therefore, there is a need in the industry to overcome some or all of the above shortcomings.